This invention relates to a composite powdered material having a matrix phase of aluminum or aluminum based alloys and a reinforcement phase of titanium diboride. This invention also relates to a process for producing the composite powdered material in which the titanium diboride is formed in situ as powders containing aluminum, titanium and boron are passed through a high temperature zone. More particularly, the high temperature zone is a plasma.
Aluminum based alloys are relatively light weight low cost materials and thus desirable for use in aerospace applications. Their use however has been limited to low temperature applications because of rapid degradation in their mechanical properties at temperatures above about 250.degree. F. Development of aluminum alloys with adequate mechanical properties at higher temperatures up to about 650.degree. F. would be highly desirable since these could be used to replace more expensive titanium based alloys.
At present, development of these high temperature aluminum alloys is based on two key concepts or technologies. These are (1) rapidly solidified alloys and (2) metal-matrix composites. The first method of rapid solidification is based on the principle that rapid cooling during the solidification process results in refined microstructures and/or supersaturation of the metal matrix with alloying elements resulting in increased precipitation hardening upon using suitable heat treatment. Atomization and melt spinning are two of the techniques used to achieve the high cooling rates. Alloying elements used to impart the desirable high temperature properties have low solubility and diffusivities in the metal matrix and precipitate as intermetallic compounds. Alloys being developed are based on the systems Al-Fe-Ce, Al-Fe-Mo, Al-Ti-Hf, Al-Cr-Zr. etc. High temperature mechanical properties of these rapidly solidified alloys is dependent on the thermal stability of the precipitated phases. Though the improvements in the high temperature mechanical properties of the advanced powder metallurgy aluminum based alloys has been impressive, they still lack specific strength equivalency with titanium based alloys.
The second method of producing high temperature high strength aluminum based materials is based on the composite approach. The reinforcement phase has high strength and high hardness and is typically an oxide, carbide, and/or a nitride. Typically these phases have very high melting points and are thermally stable in the alloy matrix. They are incorporated into the composite system by mechanical mixing with the alloy powders. Discontinuously reinforced aluminum alloys fabricated via powder metallurgy processing represent a maturing technology offering aluminum based alloys having improved specific stiffness and strength at only a slight increase in density. Silicon carbide whisker or particulate-reinforced aluminum alloys are fabricated using the composite approach. The process for fabricating whisker reinforced materials on a commercial basis has been developed by ARCO Metal's Silag Operation. A process for making particulate-reinforced aluminum alloys has been developed by DWA Composites Incorporated. It utilizes a binder to make green "pancakes" of SiC and aluminum powders which are then stacked prior to hot pressing. U.S. Pat. No. 4,259,112, Dolowy, J. F., Webb, B. A., and Suban E. C., Mar. 31, 1981. While the preliminary steps to fabricate the green shapes to be hot pressed are somewhat different, both ARCO and DWA processes involve vacuum hot pressing slightly above the solidus to achieve full densification of billet and plate. Subsequent extrusion or forging of the billet is necessary to optimize mechanical properties. The apparent need to hot press at a temperature above the solidus temperature of the alloy (that is, the alloy is partially remelted) to achieve wetting of the SiC reinforcement is a limitation of the process, since the solidification rate experienced by the remelted matrix is comparatively much slower than that of the starting powder material in the metal matrix. Thus the melting and resolidification cycle used in the process destroys the desirable rapidly solidified structure of the starting powder. The resulting alloy segregation can be deleterious in terms of the mechanical properties of the matrix and hence of the composite system.
Another composite technique called "compocasting" involves adding non-metals to partially solidified alloys. The high viscosity of the metal slurry prevents particles from settling, floating, or agglomerating. Bonding of non-metal to metal is accomplished by interaction between the respective particles. Mehrabian, R., Riek, R. G., and Flemings, M. C., "Preparation and Casting of Metal-Particulate Non-Metal Composites", Metall. Trans., 5(1974) 1899-1905 and Mehrabian, R., Sato, A., and Flemings, M. C., "Cast Composites of Aluminum Alloys", Light Metals, 2(1975) 177-193. The cooling rates experienced by the metal-matrix are again low, comparable to other casting techniques (10.sup.-3 to 1 k/s).
Still another method for producing powder metallurgy composite materials is by mechanical alloying. This is essentially a high energy ball milling operation which is done typically in a stirred ball mill called an attritor mill. High strength material results from mechanically working the alloy because of incorporation of oxides and carbides during the milling, and because of strengthening mechanisms due to severe working resulting in fine grain and sub fine grain size.
U.S. Pat. Nos. 3,909,241 and 3,974,245 relate to processes for producing free flowing powders by agglomerating finely divided material, classifying the agglomerates to obtain a desired size range, entraining the agglomerates in a carrier gas, feeding the agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles, and collecting the particles in a cooling chamber containing a protective gaseous atmosphere, wherein particles are solidified.